For a WDM transmission several channels, or several transmission signals independent of one another, are required to be sent over the same line consisting of an optical fibre by multiplexing in the domain of optical frequencies. The transmitted channels can be either digital or analog and are distinguished from each other because each of them is associated with a specific wavelength, separated from that of the other channels.
In order to enable transmission of a great number of channels using the so-called third transmission window of the silica-based fibres and the useful band in the optical amplifiers, the wavelength separation between the channels themselves is conveniently in the order of nanometres.
For a correct reception of such transmission signals, it is therefore necessary to carry out a separation between said channels, so as to convey them to the respective users.
For the purpose, low-band optical filters can be utilized through which only the selected channel can pass, so as to ensure the absence of undesired signals, that would constitute a noise, being overlapped with the selected channel.
For use of such filters, however, both a high stability in the wavelength of the transmitted signal and a high inherent stability of the passband of the filter themselves is required.
The above problem is described for example in Patent Application GB 2260046 suggesting to overlap a pilot signal with data to be transmitted, by the detection of said pilot signal the receiver being able to adjust the filter passband.
Known optical filters, in addition, suffer from shift problems, based on which a selected wavelength for the passband keeps constant only over a limited period of time after setting. Said filters, in particular in the case they are provided with piezoelectric actuators or the like, are also subjected to hysteries phenomena. Based on said phenomena, the selected value of a passband wavelength not only depends on the value of the relevant command quantity (a voltage for example), but also on the temporal law by means of which such a quantity is applied to the filters themselves.
In addition, in an optical telecommunication system the optical signal/noise ratio should be checked, at the exit of an amplification stage for example, while at the same time the same filtering conditions as applied to the receiver are being applied, so that the system functionality can be checked.
Such a check however, generally can be carried out only with the use of expensive laboratory equipments.
Patent Application GB 2272590 disdoses a method of measuring the signal/noise ratio comprising the steps of selectively filtering a transmitted signal with a notch (a narrow-band-pass filter) and measuring the signal power within said band for the filtered and unfiltered signal, in order to ascertain the signal/noise ratio in an amplifier.
Upon experiments carried out by the Applicant, however, the use of optical filters to this purpose proved to be subjected to the above problems concerning temporal stability and hysteresis and did not give appropriate results.